This invention relates to sonar devices, used on a boat to indicate the presence of sonic energy reflecting interfaces, such as those caused by the presence of fish and the water bottom. More particularly, this invention pertains to a solid state sonar apparatus for providing a printed output of the received signal resulting from the downward propagation of a high frequency sonic pulse.
A commonly employed means of measuring and indicating the depth in water below a boat of the presence of sonic energy reflecting interfaces, is a sonar apparatus. Sonar devices functions by generating sequential bursts of sonic energy into the water directed downwardly, and receiving energy reflected from intermediate interfaces of objects such as fish and also a reflection from the bottom of the water. The received energy reflected from these interfaces provides, by its travel time, means for determining the depths to the reflecting interfaces. This is the time span which elapses between the transmission of a sonic pulse until the reception of a reflected signal. The reflected signal is recorded as a trace by a moving stylus, which is controlled to move transverse to the movement of the record.
In the prior art a method frequently employed for indicating depths utilizes a rotating disc having a small gas-filled tube carried by the disc which is illuminated when a reflection echo is received. A pulse of sonic energy is transmitted at the beginning of each revolution of the disc and thereby the depths of the water down to the relecting interface in which the instrument is used is indicated by the angular displacement of the tube at the time it is flashed by the received signal.
Other devices for indicating depth includes the use of a current meter in which the depth is indicated by the magnitude of current flow through the meter. A linearly increasing current is provided and passed through the meter starting at the transmission time of the sonic pulse. The reading of the meter at the time the reflection signal is received would then be a measure of depth to that reflecting interface. This depth is proportional to the time duration required for the transmission of the sonic pulse and the received reflection.
A third type of depth indicator is a chart readout in which a moving scriber or stylus passes over a record and is energized to mark on the record in response to the received reflected signals. The markings on the record can be compared with depths indicated on the charts as a means of indicating the position or depth of the reflecting interfaces.
In the conventional sonar system in which the returned reflected signal is recorded by a stylus moving across a sheet of record paper, which is treated with a chemical substance such that when a high voltage is applied by means of a wire stylus to the paper, part of the chemical material is altered to a black color so that a black dot is printed on the sheet.
A cycle of operation of the sonar begins with a pulse generated by the movement of a magnet attached to a belt which also carries the stylus. The magnet cooperates with a pulsor coil to generate a voltage pulse. This voltage pulse is amplified and shaped and initiates the application of voltage to an oscillator and power amplifier. The high voltage, high frequency electrical pulse then operates the transducer to generate a pulse of sonic energy which goes out into the water. When this pulse of sonic energy hits a sonic reflecting surface it will generate a reflected sonic signal, which travels back to the transducer and is converted to an electrical signal. This passes to a receiver which detects and amplifies the signal, and the amplified reflection signal then goes to a high voltage source to generate the pulses for marking the records in accordance with the return of reflected signals.
Because of mechanical limitations the time of travel by the stylus across the record sheet is roughly one-third of the time for one complete cycle of the belt. Therefore, the length of time taken by the stylus to draw a single path across the record sheet is approximately one-third the period of revolution of the belt and of a single operating cycle of the sonar.
After the trace has been recorded and the stylus passes off of the record sheets, any further return pulses cannot be recorded, of course. However, a very long delayed pulse that is a reflection from a very deep bottom may come back and arrive at the transducer at slightly more than the time of one cycle of operation which would then be within the recording time of the next cycle of the operation of the sonar. Such a long delayed return of the reflected signal can be very confusing on the record and consequently causes a very serious noise problem.
There is also another noise problem. For example, when a strong reflected pulse comes back to the transmitter and to surface of the water and is again reflected downwardly to that strong reflector and back to the surface again the total delay may again be of the order of more than one operating cycle, and so this doubly reflected signal will be recorded with currently received returned signals from the second operation from the transducer.
Also under certain conditions where there is a lot of material in the water such as algae, plankton, moss and so forth, where sonic energy can be in a sense continually be reflected back and forth between these layers. The same situation may develop, namely, that a long delayed received signal will again be received in the succeeding operating cycle.
When any of these conditions occur it is important that the long delayed return signals be removed from the record in order to clarify the situation so far as the first reflected signals are concerned, which come back from reflecting interfaces which are in the zone of depth which is of principal interest to the operator of the sonar.
This invention has an important object, which is to overcome this difficulty by disabling the transmitter, detector and recorder during a second cycle of the operation and then on the third operation to enable them again to make proper record traces on every other cycle of operation.